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Smoking coil!

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AshimotoK0:
Make sure your meter is capable of accurately measuring low resistance accurately to 0.1 of an Ohm (usually 100 Ohm range) and also don't forget to short together the two probes and measure any  test lead resistance and subtract that from your readings. On some meters, like my expensive Fluke model, you can 'null' out the cable resistance. Post a pic of the one you are using for more help.

andy120t:
New battery with a full charge made all the difference. I have a good spark in the coil that wasn't smoking and nothing from the one that was. I've swapped the wiring over and the 'good' coil still gives a spark, so my wiring is OK.

I've also plugged in another coil from elsewhere and this gives a spark where the smoking coil didn't, so I'm sure that the smoking coil is scrap. Pity really, as the engine was running on all 4 cylinders, but I feel I've learnt a bit about wiring the points ignition and how they work - and importantly, what can go wrong in the absence of an 'engine run' switch which I will now be adding in.

One clarification/ question - when the points are closed, is the current just earthing through the points backplate?

Also a piccie of the coils on this engine - they look a bit precarious, is there anything I can use to hold them on .. electrical tape? Would a jubilee clip to and too metallic? All my other coils are screw-in ones and the wires look fairly secure. [ Guests cannot view attachments ]

andy120t:
Ash - Piccie of my multimeter..

[ Guests cannot view attachments ]

I think I'm getting the correct reading with that setting, but please let me know if not!
thanks

K2-K6:
"One clarification/ question - when the points are closed, is the current just earthing through the points backplate"

Yes, the primary circuit is completed by earthing through that route. With the points closed the primary coil winding induces energy to be stored in the secondary "HT" windings that are wrapped around the primary, but not connected to them. In concept like the current contactless charging systems for phones etc.

When the secondary coil holds enough energy,  the primary earth is opened with the points. At this point the energy wants to escape from it's stored state and lacking that primary route will seek any earth, which is supplied by the leads and spark plugs ultimately to give a ground route out, jumping that plug gap then gives us the spark.

There's a system backlash too, just as the plug starts to spark it comes back through the entire system to induce an escape route across the points gap but prevented from doing so by providing a buffer in the form of the condenser to absorb the shock sufficient that it's unwilling to jump the points gap. If the condenser fails, then the spark jumps the points gap and doing so terminates the one at the plug.

It's really a little bit of a magical balancing act reliant on helpful potential faults and useful characteristics being arranged in balance over the whole system.

McCabe-Thiele (Ted):

--- Quote from: K2-K6 on June 16, 2021, 10:39:32 AM ---"One clarification/ question - when the points are closed, is the current just earthing through the points backplate"

Yes, the primary circuit is completed by earthing through that route. With the points closed the primary coil winding induces energy to be stored in the secondary "HT" windings that are wrapped around the primary, but not connected to them. In concept like the current contactless charging systems for phones etc.

When the secondary coil holds enough energy,  the primary earth is opened with the points. At this point the energy wants to escape from it's stored state and lacking that primary route will seek any earth, which is supplied by the leads and spark plugs ultimately to give a ground route out, jumping that plug gap then gives us the spark.

There's a system backlash too, just as the plug starts to spark it comes back through the entire system to induce an escape route across the points gap but prevented from doing so by providing a buffer in the form of the condenser to absorb the shock sufficient that it's unwilling to jump the points gap. If the condenser fails, then the spark jumps the points gap and doing so terminates the one at the plug.

It's really a little bit of a magical balancing act reliant on helpful potential faults and useful characteristics being arranged in balance over the whole system.

--- End quote ---

Sometimes Back EMF has it's uses !
I was going to say a positive side but thought that was too contrived. :)

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